1. Field of the Invention
The invention pertains to the field of integrated, electrically operable micro-valves and, more particularly, to the field of low leak rate integrated micro-valves for industrial, corrosive and ultra-clean applications.
2. Description of the Related Art
Micromachined integrated valves are known in the prior art. Examples of various embodiments of such normally open valves are given in U.S. Pat. Nos. 4,821,997 and 4,824,073 and 4,943,032 and 4,966,646 by Mark Zdeblick, assigned to Stanford University, hereinafter referred to as the "Zdeblick" patents, the disclosures of which are hereby incorporated by reference. Such valves generally include a three tier structure which uses the top two tiers to form a sealed cavity with a fluid, having a low boiling point and/or high expansion coefficient, trapped therein, and having one wall of that cavity formed as a thin, flexible membrane. The top two tiers can be silicon, quartz or glass substrates, or any other appropriate material. Typically at least one tier or substrate is silicon to take advantage of silicon micromachining techniques to form the cavity and flexible membrane. The valves of the "Zdeblick" patents also include a resistive element formed on an interior surface of the sealed cavity. This element has electrical connections to a current source to provide for an electrical input through the resistor causing a resultant heating effect. The bottom tier typically has a valve seat and port formed therein. In this manner, fluid flow can be halted by closing the valve port to stop flow across the valve seat and through the port. In the "Zdeblick" patents, when current is passed through the resistor, the trapped fluid is heated causing the flexible membrane to flex far enough to come into contact with the valve seat formed of the lower body. Thus fluid flow is cut off between the input channel and the output channel through the port.
Normally open valves are suitable for some applications, but in other applications normally closed valves are needed, that is valves wherein no flow from the input channel to the output channel occurs in the de-energized state. One example of such a normally closed valve is the Fluistor.TM. (trademark of Redwood Microsystems, Inc. of Menlo Park, Calif.) Microvalve (NC-105) manufactured by Redwood Microsystems, Inc. Referring to FIG. 1, a simplified representation of this valve 5 in the de-energized state is shown. In the Fluistor.TM. valve 5 a flexible membrane 20 is formed from a middle layer or substrate 14. Flexible membrane 20 also serves as a wall of a cavity 26 formed between the middle substrate 14 and an upper layer or substrate 16. Cavity 26 has a resistive element or heater 30 formed therein to provide for heating a fluid 28 (represented by "squiggly" lines) that is sealed within cavity 26. Fluid 28 is selected so that when heated it expands to cause flexible membrane 20 to flex or move. This movement is coupled to a lower layer or substrate 12 through a mechanical coupling or pedestal 22. Where substrate 12 is fixed, as in the Fluistor.TM. NC-105, movement of flexible membrane 20, coupled with the placement of pedestal 22, results in middle and upper substrates 14 and 16, respectively, moving away from lower substrate 12. Thus, substrate 12 separates from middle substrate 14, and an outlet port 44, formed through substrate 12 is unblocked or opened at a valve seat region 40. In this manner, the outlet port 44 is opened and placed in fluidic communication with an inlet port 42.
The Fluistor.TM. valve works well for controlling the flow of non-corrosive fluids and/or where leak rates of no less than approximately 1.times.10.sup.-4 cc-Atm/sec of Helium (cubic centimeter-Atmospheres per second as calibrated using helium) are required. However, it is not designed for applications requiring the control of corrosive fluids and/or applications requiring leak rates of 1.times.10.sup.-6 cc-Atm/sec of Helium or less. In addition, like the valves of the "Zdeblick" patents, the Fluistor.TM. valve uses the material of flexible membrane 20, or a contiguous extension thereof, to directly seal port 44. Such direct use of membrane 20 limits the design possibilities of valve 5.
Accordingly, there is a need for an integrated, micro-valve, which can be used to control corrosive fluids. There is also a need for an integrated, micro-valve that can achieve a leak rate of 1.times.10.sup.-6 cc-Atm/sec of Helium or less. In addition, there is a need for an integrated, micro-valve that can control corrosive fluids while achieving a leak rate of 1.times.10.sup.-6 cc-Atm/sec of Helium or less. Finally there is a need for normally open and normally closed micro-valves that provide for the above mentioned needs without using the flexible membrane as an essentially direct sealing device for the valve port; in this manner, increasing the design choices and range of potential applications for the micro-valve.